feat(agent): Agent 工作台 - 计划插话记忆、统一模型选择器并修复摘要与视频入库

将计划清单、运行中插话和个人记忆接入云端 Agent；全站模型选择器统一为左右二级菜单。画布摘要按预算截断不再整轮拒绝，首页视频未知宽高可入库。README 补充贡献者 morrow。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/app/cloud_agent_working_context_test.go

@@ -0,0 +1,53 @@
+package app
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestTrimCloudAgentTextHistoryKeepsRecentRounds(t *testing.T) {
+	history := make([]providerTextMessage, 0, 36)
+	for round := 1; round <= 12; round++ {
+		history = append(history,
+			providerTextMessage{Role: "user", Content: "目标" + strconv.Itoa(round)},
+			providerTextMessage{Role: "assistant", Content: "回复" + strconv.Itoa(round)},
+			providerTextMessage{Role: "user", Content: "上一轮已结束（completed）。本轮只执行当前用户消息，不要回头核对上一轮待办是否完成。"},
+		)
+	}
+	trimmed := trimCloudAgentTextHistory(history, 10, cloudAgentHistoryMaxBytes)
+	if cloudAgentHistoryUserInstructionCount(trimmed) != 10 {
+		t.Fatalf("应只留最近 10 轮用户目标，got %d / %d msgs", cloudAgentHistoryUserInstructionCount(trimmed), len(trimmed))
+	}
+	if trimmed[0].Content != "目标3" {
+		t.Fatalf("最旧保留轮次不对：%q", trimmed[0].Content)
+	}
+	last := trimmed[len(trimmed)-1]
+	if last.Role != "user" || last.Content != "上一轮已结束（completed）。本轮只执行当前用户消息，不要回头核对上一轮待办是否完成。" {
+		t.Fatalf("最近一轮摘要应还在：%+v", last)
+	}
+}
+
+func TestTrimCloudAgentTextHistoryDropsOldestToFitBytes(t *testing.T) {
+	history := []providerTextMessage{
+		{Role: "user", Content: "旧目标" + string(make([]byte, 40000))},
+		{Role: "assistant", Content: "旧回复"},
+		{Role: "user", Content: "新目标"},
+		{Role: "assistant", Content: "新回复"},
+	}
+	trimmed := trimCloudAgentTextHistory(history, 10, 8000)
+	if cloudAgentHistoryUserInstructionCount(trimmed) != 1 || trimmed[0].Content != "新目标" {
+		t.Fatalf("超字节时应丢掉旧轮：%+v", trimmed)
+	}
+}
+
+func TestTrimCloudAgentTextHistoryCannotHideSingleOversizedRound(t *testing.T) {
+	history := []providerTextMessage{
+		{Role: "user", Content: strings.Repeat("x", 70000)},
+		{Role: "assistant", Content: "ok"},
+	}
+	trimmed := trimCloudAgentTextHistory(history, 10, cloudAgentHistoryMaxBytes)
+	if cloudAgentHistoryJSONSize(trimmed) <= cloudAgentHistoryMaxBytes {
+		t.Fatal("只剩一轮且本身超过 64KB 时不能假装压进去")
+	}
+}